Lasers are used for a variety of machining operations including drilling. The laser may be directed and/or focused at the surface of a substrate until the laser energy melts or ablates the material to form a hole. In some drilling applications (e.g., in certain aerospace applications), the substrate being drilled is positioned in close proximity (e.g., less than 1/10 in.) to another substrate or structure. In such applications, back-wall strike by the laser after penetrating the substrate being drilled has always been a problem.
A conventional laser beam with a Gaussian distribution of laser energy becomes very hot at the center of the beam, which breaks through the substrate first. After the center of the beam breaks through, this hot central portion of the beam may cause damage to the back wall while the laser beam continues to drill the hole of the desired diameter. Some unsuccessful attempts at avoiding back-wall strike have included the use of cameras to monitor the drilling for determining when the laser penetrates the substrate being drilled.